The production of chlorine by the electrolysis of aqueous solutions of halides in a cell comprising a permselective membrane having a conductive, catalytic electrode permanently affixed to the surface thereof and in physical contact with an external electron current distributor is described in an Application for U.S. Letters Patent Ser. No. 000,491, filed Jan. 2, 1979, now U.S. Pat. No. 4,247,376, issued Jan. 27, 1981 a division of Ser. No. 866,299, filed Jan. 3, 1978, now abandoned, in the names of Russell Mason Dempsey et al assigned to the General Electric Company, the assignee of the present invention. Cells of this type include a graphite electron current distributor which contacts the surface of the catalytic electrode bonded to the membrane. Such electron current collectors have a plurality of elongated, continuous, parallel ribs extending from a conductive base. The elongated ribs contact the catalytic electrode to distribute current while the channels formed between the ribs provide fluid distribution channels for the anolyte entering the cell as well as for gases evolved at the electrode. Continuous ribs may have the disadvantage of obstructing a relatively large area of the electrode thereby limiting access of the aqueous anolyte to the electrode at these locations. Because of the obstruction or "blinding" of areas of the bonded electrode, chloride ion starvation under these areas can result in electrolysis of water and the evolution of oxygen.
Coevolution of oxygen at a chlorine anode has a number of practical consequences, all of them undesirable. The evolution of oxygen, of course, represents a process inefficiency and increases the electrical cost necessary for producing chlorine, i.e., increases the production cost of chlorine. High oxygen levels also result in severe corrosion of graphite cell components. As pointed out previously, in an HCl electrolysis cell having anode electrodes bonded directly to an ion exchanging membrane, the preferred current collector fluid distributor is a molded graphite-polymer-aggregate having a plurality of parallel grooves which contact the electrode provide current to the electrodes while distributing the anolyte and the electrolysis product at the anode. If the oxygen level in the chlorine remains below 0.1% (V/V) very little corrosion of the graphite is observed whereas levels in excess of 1% (V/V) lead to severe corrosion in a matter of days. It is therefore highly desirable to maintain the oxygen level at or below 0.1%.
The discharge potential of oxygen, i.e. the standard electrode potential for O.sub.2, is actually lower than that of chlorine (1.23 volts v. 1.36 volts). However, the great irreversibility of oxygen electrodes (i.e. the overpotential for oxygen) permits preferential evolution of chlorine despite these thermodynamic considerations. Thus, normally, chlorine is evolved preferentially although oxygen evolution is not entirely suppressed. The oxygen evolution reaction can be inhibited by maintaining a high acid concentration at the electrode reaction size. By maintaining the chloride ion concentration sufficiently high, chlorine discharge at the anode is facilitated.
The concentration of hydrochloric acid at any reaction site can be defined by the expression: ##EQU1## where i=the cell current density
F=FARADAY PA1 D=the HCl diffusion coefficient PA1 C.sub.S =the HCl concentration of the feed PA1 C.sub.R =the HCl concentration at the discharge site underneath the contact point between the electrode and the current collector PA1 l=the true diffusion path length for the HCl to the discharge site.
Thus it may be seen the the acid concentration at the reaction site, and hence the O.sub.2 in Cl.sub.2 level, is a function of both the diffusion path length and the feed stock acid concentration.
The discharge site below a contact element of the current collector has a longer diffusion path than do the sites below the liquid flow channel because the acid must diffuse laterally underneath the contact element and within and across the anode thickness to reach the electrode reaction site below a contact element, while the acid need only be diffuse across the anode thickness to reach the discharge site below the flow channel. Because of the greater path length, the acid concentration below the contact element is reduced and the rate of O.sub.2 evolution tends to increase. By increasing the acid concentration of the feed, (C.sub.S) the chloride ion content beneath the contact element is maintained sufficiently high to minimize oxygen evolution even though the area is partially obstructed or "blinded". Thus, it is customary to operate with anolyte acid feed concentration in excess of 10 M, preferably between 10 and 12 M to maintain the oxygen content in the chlorine at 0.1% or less.
While maintaining the feed stock acid concentration at very high levels is effective in reducing the oxygen evolution reaction, it has been found that it does have a number of shortcomings which make it less than an optimum solution. The problem is that the vapor pressure of hydrogen chloride, which is a gaseous material, is both a function of temperature and concentration. Its solubility in water is a logarithmic function of temperature. Operation of the cell at temperatures of 60.degree. C. and above (which is desirable to minimize internal resistance, and electrode overpotential and maximize electrical efficiency) results in high hydrogen chloride partial pressures and impure chlorine product which must be purified. This expense is, of course, in addition to the undesirable corrosive effects of the hydrogen chloride on the cell and downstream equipment. If the operating temperature of the cell is reduced (i.e., to 30.degree.-40.degree. C.) in order to maintain the hydrogen chloride vapor pressure at a reasonable level, the overpotential of the electrodes and the internal ohmic loss increases and the power efficiency of the system decreases also resulting in higher chlorine production costs.
Thus, presently known techniques to minimize oxygen evolution in chlorine cells by increasing the acid feed stock concentration result in high hydrogen chloride vapor pressures; in an impure chlorine product, in added expense due to purification costs and in potential-corrosion of equipment. Attempts to control the hydrogen chloride vapor pressure when operating with a high feed acid concentration by reducing the temperature does reduce the vapor pressure, but results in a substantial penalty in cell voltage because internal resistance and the electrode overpotential increases as the temperature is reduced.
Applicant has now found that it is possible to electrolyze aqueous hydrogen chloride with low oxygen evolution levels (less than 0.1%), at high temperatures (60.degree.-80.degree. C.) with low hydrogen chloride vapor pressures (less than 0.1 of an atmosphere, i.e. 76 torr) by maintaining high chloride ion concentrations at the membrane electrode current collector contact area even with low feed acid concentration, i.e. acid concentration of less than 9 M and preferably 8.5 or 8 M or less.
Applicant has found that this may be achieved by minimizing the obstructed or "blinded" areas of the electrode and by maximizing diffusion of chloride to the "blinded" areas. To this end, a novel current collector construction is provided in which a array of contact elements are utilized in place of continuous parallel contact elements to establish a planar array of individual, unconnected current transfer areas, preferably in the plane of the electrode. The incoming anolyte is broken up into a plurality of intersecting anolyte streams which flow across the electrode surface. The turbulent flow due to the intersecting streams surround the point contact elements, and the increased perimeter exposed to anolyte permits diffusion of the acid anolyte to take place from all sides of the contact elements. This reduces the diffusion path length so that the ion chloride concentration beneath the contact may be maintained at a sufficiently high level to reduce oxygen evolution below 0.1% with feed acid concentrations of less than 9 molar. Because the cell is operated with lower feed acid concentrations, the cell may be operated at a much higher temperature (.ltoreq.60.degree. C.), without raising the hydrogen chloride vapor pressure to an undesirable level, (i.e. the vapor pressure is maintained at 0.1 atmosphere --76 torr or less). By operating at temperatures of 60.degree. C. and above the cell voltage is substantially reduced because at these temperatures the electrode overpotential and ohmic losses are substantially reduced.
It is therefore a principal objective of this invention to provide a method and apparatus for electrolyzing aqueous halides in which coevolution of oxygen at the anode is minimized while operating at temperatures which maximize the cell efficiency.
Yet another objective of the invention is to provide a process for generating halogen from an aqueous hydrogen halide in which the vapor pressure of the halide is minimized at temperatures at which the cell is most efficient;
Yet another objective of the invention is to provide a process for generating chlorine from a hydrogen chloride with minimal coevolution of oxygen at low concentrations of the hydrogen halide anolyte;
Still another objective of the invention is to provide a cell for generating chlorine from aqueous hydrogen chloride in which the oxygen content of the chlorine is 0.1% or less and the hydrogen chloride vapor pressure is very low at cell operating temperatures of 60.degree. C. or more;
Other objectives and advantages of the invention will become apparent as the description thereof proceeds.
In accordance with the invention, halogens such as chlorine, bromine and etc. are generated by the electrolysis of aqueous hydrogen halides at the anode of an electrolysis cell which includes an ion exchange membrane separating a cell into catholyte and anolyte chambers. A thin, porous, gas permeable catalytic anode is maintained in intimate contact with the ion exchange membrane bonding it to the surface of the ion exchange membrane. A graphite electron current conducting distributor which includes a planar array of conductive projections contacts the bonded electrode at a plurality of points either directly or through the medium of an interposed conductive screen. By virtue of the multipoint contact array configuration, turbulent flow is established over the electrode as the incoming anolyte is divided into in a plurality of intersecting streams. The multiple anolyte streams surround the contact elements and maximize diffusion of the anolyte beneath the surface and into contact with and into the chlorine evolving bonded electrode. In effect the diffusion path of the current conductor is decreased thereby maintaining the chloride ion concentration beneath the contacting element such that coevolution of oxygen is held at 0.1% by volume or less with low feed acid concentrations. The vapor pressure of the hydrogen chloride above the fluid is kept very low (less than 0.1 atmospheres) while operating the cell at 60.degree. C. or above so that the overpotential of the electrode for chlorine evolution and separator resistance are minimized.